1. Field of the Invention
The present invention relates to a vacuum switching tube having coaxial contacts with a shield therearound for generating an axial magnetic field.
2. Description of the Prior Art
A vacuum switching tube is disclosed in German Patent OS No. 24 42 936 and corresponding U.S. Pat. No. 3,899,079 wherein it is proposed to generate an axial magnetic field from an ion current present at the discharge space between the two electrodes. A screen lying at the potential of one of the two electrodes is used for the generation of the axial magnetic field. In practice, the ion current amounts to about 10% of the overall current.
Since the screen in the known vacuum switching tube lies at the potential of one of the two electrodes, an asymmetrical distribution of potential is present. This asymmetrical potential distribution requires a considerably larger space between the non-contacted cooperating electrode and the screen than that for potential-free arrangements of shielding which are found in switching tubes without axial magnetic fields.
In the literature is known numerous executions of switch contacts in a switching tube containing means for generating an axial magnetic field. These contacts must generate the full required magnetic field and, therefore, are heavier than simple switch contacts, on the one hand, and, on the other hand, require additional space due to the means for generating the axial magnetic field. Based on the design of the contacts, this space can be required in the axial direction and/or in the radial direction. A considerable enlargement and a resulting increase in price of the switching tubes is caused in both instances. Furthermore, an added expenditure for the drive energy in the vacuum switch and an undesirable rise in the resistance of the internal contacts and, thus, the overall tube resistance due to the coil-like current paths required for generating the axial magnetic field derives. The increased resistance, in turn, increases the dissipated power at nominal current when the contacts are in the engaged position.